This invention relates generally to gas turbine engines, and more particularly, to methods and apparatus to control gas turbine engine rotor assembly tip clearances during transient operation.
Gas turbine engines typically include an engine casing that extends circumferentially around a compressor, and a turbine including a rotor assembly and a stator assembly. The rotor assembly includes at least one row of rotating blades that extend radially outward from a blade root to a blade tip. A radial tip clearance is defined between the rotating blade tips and a shroud attached to the engine casing.
During engine operation, the thermal environment in the engine varies and may cause thermal expansion or contraction of the rotor and stator assemblies. This thermal growth or contraction may or may not occour uniformly in magnitude or rate. As a result, inadvertent rubbing between the rotor blade tips and the engine casing may occur or the radial clearances may be more open than the design intent. Continued rubbing between the rotor blade tips and engine casing may lead to premature failure of the rotor blade or larger clearances at other operating conditions which can result in loss of engine performance.
To facilitate optimizing engine performance and to minimize inadvertent rubbing between the rotor blade tips and the engine casing, at least some known engines include a clearance control system. The clearance control system supplies cooling air to the engine casing to control thermal growth of the engine casing to facilitate minimizing inadvertent blade tip rubbing. The case is heated and cooled by air coming from fan, booster, or compressor compressor bleed sources, which may cause the case to shrink or expand due to changes in temperature.
Some known clearance control systems account for disk elastic deflection and blade thermal growth from idle conditions to aircraft take-off by having a large clearance at idle in order to prevent blade tip rubs later in the engine cycle. These systems require a large change in temperature at steady-state conditions to reduce clearance to a minimum level. Typically, the temperature change of the case that is necessary to reduce the steady-state clearance is beyond the capability of these systems.